1. Field of the Invention
This invention refers to a photonic sensing method and device for the detection of very small sized substances. More particularly, the invention is based on the physical properties of periodic dielectric structures of photonic forbidden band, over which the substances object of the sensing are placed.
2. Description of the Related Art
Nowadays, it is essential to have methods or devices for sensing that allow fast, efficient and reliable measurement of a large number of parameters, such as, temperature, pressure, electric field intensity, etc. In addition to the measurement of the aforementioned physical magnitudes (for which there already exist rather mature measurement methods), the main interest currently centers on the development of sensing devices or methods for the detection, identification and quantification of substances (which we shall call analyte) such as gas, liquids, proteins, hormones, bacteria, or DNA, amongst many others. These sensing devices and methods have applications in many fields, such as, pharmaceutical research, disease diagnosis, pollutant detection or the bacteriologic war.
The methods typically used for analyte detection are based on the use of so called markers. Through an adequate treatment of the sample containing the analyte to be detected (generally this method is carried out in a laboratory), a link of the marker used to the analyte is achieved. This marker consists of a material featuring specific physical properties, such as, fluorescence, radioactivity, etc., so that detection of the analyte is carried out indirectly, measuring the fluorescence, radioactivity, etc. in the sample containing the analyte. However, the use of markers to carry out sensing presents various problems, such as could be the need to do previous preparation of the sample to be analyzed (which may be complex and require a large amount of time) or the difficulty to find a method which sets the markers solely and specifically to the analytes we wish to detect.
It is, therefore, of great importance to have sensing devices or methods that enable the detection without the need to use markers. One of the options that currently raise the most interest is the use of photonic devices to carry out sensing functions. A photonic device is defined as one in which the frequency of spread signals is within the optical range of the spectrum. This type of devices have a certain material distribution with a certain refraction index n, so that the device's response is given by the refraction index of the materials comprising it, as well as by the shape that these materials feature in the structure. An example of a very common photonic device nowadays is fiber optic, which is a means of transmission in which there is a circular nucleus made of a material with a refraction index n1 surrounded by a coating of a material with a refraction index n2. Since the refraction index of the nucleus is higher than that of the coating (n1>n2), the light will remain confined to the nucleus due to the total internal reflection phenomenon and it will be able to propagate through the fiber. The fiber's frequency response will depend on various factors, such as the contrast of indices between the nucleus and the coating, the diameter of the nucleus, the coating's thickness, fiber's imperfections, etc.
In the case of fiber optic, the contrast between the nucleus and the coating indices is very small (under 1%), which causes the structure's nucleus to have a diameter of a few dozen microns. When the contrast of indices between the guide's nucleus and the material forming the coating increases, the confinement of the field in the high index region forming the guide's nucleus increases, therefore being able to significantly reduce the size of the devices. This is what is known as nanophotonics, where the use of materials with a high refraction index allows attainment of devices with a size in the hundreds of nanometers.
The field of nanophotonics has experienced significant evolution in the last few years, during which several structures have been developed for the implementation of functionalities in the optical domain, such as guiding, WDM filters, channel addition/extraction filters, commuters, etc. A great part of this evolution has been due to the possibility of using planar substrate structures, through which photonic circuits are created in flat layers of a certain material, with a certain thickness (e.g. Silicon, Silicon oxide, Silicon nitride, etc.), thus leading to integrated photonics. Precisely, when the materials used to create the integrated photonic circuits are compatible with those used in the microelectronics industry (e.g. Silicon, Silicon nitride, etc.), it is possible to make direct use of the manufacturing processes coming from that industry, thus enabling attainment of products with a reduced cost and apt for mass manufacturing.
The use of photonic devices to carry out sensing functions has been demonstrated by several research groups. The transduction technique used to carry out the sensing of those types of structures is the variation of the refraction index. As previously described, the response of a certain photonic device depends both on its shape or dimensions as well as the refraction index of the materials forming the device. This way, when the analyte to be detected causes a change in the refraction index of the structure over which it is placed (generally in the region above the guided element of the signal), this change may be detected through variation of the photonic device's response.
There are various configurations for photonic devices to carry out sensing functions. Currently, the most popular are based on resonance structures, such as disks or rings, which are coupled to an access guide (K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, and R. Baets, “Silicon-on-Insulator microring resonator for sensitive and label-free biosensing,” Opt. Express 15, 7610-7615 (2007) and C. A. Barrios, K. B. Gylfason, B. Sanchez, A. Griol, H. Sohlström, M. Holgado, and R. Casquel, “Slot-waveguide biochemical sensor,” Opt. Lett. 32, 3080-3082 (2007)). This type of structures behaves as a cavity, so that only those modes whose wavelength fulfils the condition of resonance may exist in its interior. These wavelengths are extracted from the access guide, thus observing peaks rejected in its transmission spectrum. The position of the resonances depends on the refraction index of the material forming the structure. When making the detection, the substance to be detected is placed over the resonance structure, so that the variation in the refraction index of the material surrounding the structure causes certain displacement of the resonance position. This variation in wavelength of the resonance is used to determine the refraction index of the material placed over the structure. If carrying out sensing of a specific analyte (as opposed to detection of a homogeneous substance with a certain refraction index), the displacement of resonances indicates the quantity of analyte present in the fluid. Using this type of sensors, very high sensibilities have been demonstrated, attaining values over 200 nm/UIR (Refraction Index Unit) (K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, and R. Baets, “Silicon-on-Insulator microring resonator for sensitive and label-free biosensing,” Opt. Express 15, 7610-7615 (2007) and low detection limits of around 2×10-5 UIR (C.-Y. Chao, W. Fung, and L. J. Guo, “Polymer Microring Resonators for Biochemical Sensing Applications,” IEEE J. Sel. Top. Quantum Electron. 12, 134-142 (2006).
Another type of photonic structures of great interest for the development of sensing devices are the periodic dielectric structures (occasionally also known as photonic crystals). This type of periodic structures, if an adequate design of the structure is executed (i.e. periodicity, size of elements forming the periodic structure, etc.) and the contrast of the refraction indices of material used to create the structure is sufficient, the so called forbidden photonic bands may appear (usually this region is known by its English term photonic band gap): frequency regions of the transmission spectrum in which propagation of the wave is not permitted (J. D. Joannopoulos, R. D. Meade, and J. N. Winn, Photonic Crystals: Molding the Flow of Light (Princeton U. Press, Princeton, N.J., 1995)). As with the case of resonant structures, the position of the forbidden photonic band also varies when the refraction index of the material comprising the structure is modified. This way, when placing a certain substance over the periodic structure, the displacement experienced by the forbidden photonic band may be used to determine the substance's refraction index (or the quantity of certain analyte present in the fluid). Instead of directly using the complete periodic structure, lineal or punctual defects may be introduced in order to create guides or cavities. In this case, what is used to carry out detection is the position of the guided mode (in the case of the guide) or the resonant mode (in the case of the cavity), in a way similar to the one described for the case of the complete periodic structure. The benefits achieved using this type of structures are not as high as those achieved using resonant structures, although a higher evolution of this technology is expected, allowing attainment of better results. The values currently demonstrated are sensibilities of around 65 nm/UIR and detection limits of around 1×10-4 UIR (N. Skivesen, A. Têtu, M. Kristensen, J. Kjems, L. H. Frandsen, and P. I. Borel, “Photonic-crystal waveguide biosensor,” Opt. Express 15, 3169-3176 (2007)).
Both types of devices described above (resonant structures and dielectric periodic structures) base detection on the variation produced on the spectrum, either by wavelength displacement of the resonances, the photonic forbidden band, or the guided modes. There are, additionally, other integrated photonic structures allowing the execution of sensing functions directly using a signal's output amplitude of a certain wavelength, so that it is not necessary to carry out a scan providing the device's transmission spectrum. Examples of this type of sensing systems, based on amplitude, include the Mach-Zehnder interferometers (Th. Schubert, N. Haase, H. Ktick, and R. Gottfried-Gottfried, “Refractive-index measurements using an integrated Mach-Zehnder interferometer,” Sens. Actuators A Phys. 60, 108-112 (1997)) or the directional couplers (B. J. Luff, R. D. Harris, J. S. Wilkinson, R. Wilson and D. J. Schiffrin, “Integrated-optical directional coupler biosensor,” Opt. Lett. 21, 618-620 (1996)). Even though these systems are able to prevent the need to execute a frequency scan, providing simpler sensing devices, they have the problem that they require very large device dimensions (of a few millimeters) in order to have a sufficiently large interaction between the field and the analyte for detection, since they are only comprised of a dielectric guide through which the signal spreads, without producing an effect that allows an increase of the signal's interaction with the analyte. Therefore, due to this size, it is not possible to attain final devices with a high level of integration.
The photonic sensing method and device executed according to this invention is framed within the photonic sensors based on the direct measurement of the output amplitude, like those devices reported on (Th. Schubert, N. Haase, H. Ktick, and R. Gottfried-Gottfried, “Refractive-index measurements using an integrated Mach-Zehnder interferometer,” Sens. Actuators A Phys. 60, 108-112 (1997) and B. J. Luff, R. D. Harris, J. S. Wilkinson, R. Wilson and D. J. Schiffrin, “Integrated-optical directional coupler biosensor,” Opt. Lett. 21, 618-620 (1996)). However, unlike the technical solutions in those references, this invention features numerous advantages when implementing sensing systems, most of them derived from the small size of the dielectric structures utilized, which allows for an efficient interaction between those structures and the analytes to be sensed.
In addition to enabling a better interaction between the sensor and the analyte, the fact of having a reduced size facilitates, additionally, attainment of a high level of integration of the final device, which allows the use of a large number of sensitive photonic structures in a very small area, thus being able to carry out multiple simultaneous detections (e.g. for multi-analyte measurements, comparative analysis, etc.).
The possibility of using small-sized periodic structures makes it possible, additionally, for only very small substance volumes to be necessary (in the range comprised between milliliters and microliters, in the case of liquids, or micrograms and femtograms in the case of solids) to carry out detection of the analyte. An example of an embodiment of great interest would be that in which a detection of one (or several) substances is carried out in the blood using only a single drop of the fluid.
Another advantage of this invention with respect to the devices reported on [6] and [7], is that there is no need to use the guiding structures featured by those devices, either through index contrast guides or introducing lineal defects in the periodic structure. This need of guidance makes it, in practice, tremendously difficult to apply those techniques to tri-dimensional periodic structures, such as opals. Furthermore, large scale production of this type of devices would effectively be hardly attainable, since they do not allow for the use of manufacturing techniques from microelectronics. This invention, however, describes a photonic sensor that does not require the use of any guiding technique which naturally allows its application to tri-dimensional structures.
The design of the present invention, therefore, facilitates for the analytes to be sized under one nanometer, through a direct measurement of the sensor's output amplitude, thus preventing the need to obtain the spectral response from the device, which facilitates a simpler, more-direct sensing mechanism, applicable to all types of periodic dielectric structures.